The present invention relates to a drive circuit for driving a group of driven elements arranged in an array pattern, a light print head including the drive circuit; and an image forming apparatus including the light print head.
In a conventional image forming apparatus such as a printer of an electro-photography type, light is selectively irradiated a charged photosensitive drum according to print information to form a static latent image. Then, toner is attached to the static latent image for developing and forming a toner image, and the toner image is transferred and fixed to a sheet. In the conventional electro-photography type printer, in addition to a light emitting diode (LED), a light emitting thyristor is used as a light source.
When the light emitting diode is used as a light source, a drive circuit is disposed to correspond to the light emitting element with each other. The light emitting element is switched between a turned-on state and a turned-off state when a current directly flows or stops flowing between an anode terminal and a cathode terminal thereof.
On the other hand, when the light emitting thyristor is used as a light source, a drive circuit is disposed to correspond to an N number of the light emitting elements (N>1). In this case, a gate terminal thereof is used for selecting the light emitting element to emit light, and a current flows between an anode terminal and a cathode terminal thereof to adjust light emitting power.
Patent Reference has disclosed a light source using the light emitting thyristor with a configuration called a self-scanning LED. In this case, the light emitting thyristor performs a latch operation, so that a shift register operation is performed with a two-phase clock signal. Further, an anode of the light emitting thyristor is driven through a current regulating effect of a constant voltage power source output from a CMOS inverter and a series resistor (refer to patent Reference).    Patent Reference: Japanese Patent Publication No. 2004-195796
As described above, in the drive circuit of the light emitting thyristor, the anode of the light emitting thyristor is driven through the current regulating effect of the constant voltage power source output from the CMOS inverter and the series resistor. In the configuration, when a voltage of the power source varies, a drive current varies significantly. Accordingly, an image forming apparatus using the configuration may form an image with an uneven density, thereby lowering print quality. Further, the resistor needs to be connected in series for regulating a current. Accordingly, it is difficult to turn on and off at a high speed due to a long rise time and a long fall time of a drive current, thereby making it difficult to increase an operational speed.
In view of the problems described above, an object of the present invention is to provide a drive circuit capable of solving the problems of the conventional drive circuit. A further object of the present invention is to provide a light print head including the drive circuit, and an image forming apparatus including the light print head.
In the present invention, it is possible to reduce a variance in a drive current due to a variance in a power source voltage or a manufacturing variance of a light emitting element. Accordingly, it is possible to reduce a density fluctuation in the image forming apparatus using the light emitting element, thereby improving print quality. Further, it is possible to turn on and off at a high speed through shortening a rise time and a fall time of a drive current, thereby making it possible to increase an operational speed.
Further objects and advantages of the invention will be apparent from the following description of the invention.